The present invention relates to burst noise suppression in digital communications and, more particularly, to an improved method of chip interleaving in direct sequence spread spectrum (DSSS) modulation.
DSSS is a well-known method of suppressing narrow band noise in a digital communications channel. See, for example, Robert C. Dixon, Spread Spectrum Systems (John Wiley & Sons, New York, 1984). One familiar example of DSSS is the CDMA protocol of cellular telephony. See, for example, Andrew J. Viterbi, CDMA: Principles of Spread Spectrum Communications (Addison Wesley, Reading Mass., 1995) and Ramjee Prasad, CDMA for Wireless Personal Communications (Artech House, Norwood Mass., 1996). In DSSS, a message string of data bits is modulated by a pseudorandom binary code sequence to produce the string actually transmitted. If the original message string contains M bits, and each bit is modulated with N chips of the pseudorandom binary code sequence, the resulting chip string contains MN chips, with N sequential chips corresponding to each of the original M bits. At the receiver, the original message string is recovered by multiplying the received chip string by the same binary code sequence as was used to modulate the data bits, thus recovering each bit from the corresponding sequential group of N chips.
Although DSSS provides resistance to narrow band noise, it is vulnerable to time-limited noise bursts. Therefore, the technique of chip interleaving has been developed, to render DSSS signals resistant to both narrow band noise and burst noise. In chip interleaving, one chip is selected from each modulated data bit, and the M chips thus selected are combined to form a chip frame. A total of N chip frames are assembled, each with M chips, each with a different chip from each of the modulated data bits. The N chip frames thus formed constitute, in sequence, the transmitted string, also referred to herein as a "packet". In this way, each data bit is spread over the entire transmitted string instead of being concentrated at one position in the string. At the receiver, the chip selection is inverted to recover the non-interleaved string, which then is multiplied by the binary code sequence as in standard DSSS.
Several methods of chip interleaving are known in the art. Tachikawa et al. (Shin-ichi Tachikawa, Kiyoshi Toda, Takehiro Isikawa and Gen Marubayashi, Direct Sequence/Spread Spectrum Communications System Using Chip Interleaving and its Applications for High-Speed Data Transmissions on Power Lines, Electronics and Communications in Japan, Part 1, Vol. 75, No. 4, pp. 46-58 (1992)) use pseudorandom sequences to randomly distribute the M chips of each chip frame. Olmstead (U.S. Pat. Nos. 5,274,667 and 5,335,247) forms a large number of sequentially interleaved chip frames of length M directly, by modulating the data string with a long pseudorandom number, and then pseudo-randomizes the order of the chips in each frame. The chip frames are transmitted until the receiver returns an acknowledgment of having received an uncorrupted message.